The present invention relates generally to an apparatus and method for detecting wear in a material processing machine and, in particular, to an apparatus and method for determining the wear of feedscrew flights and adjacent barrel interior surfaces.
Typically, a barrel and feedscrew are used to mix and melt various materials used in extrusion, blowmolding and injection processes. For example, bulk plastic material is fed into an extruder, an injection molding machine or a blow molder through the barrel or cylinder utilizing a rotating helical screw. Such a machine is operated at an elevated temperature and if it is required to be shut down, the plastic material tends to solidify. Many plastic materials are highly abrasive and tend to wear the flights on the screw thereby widening the gap between the outer edge of the flights and the inner surface of the barrel or cylinder. Such a condition will tend to prevent the uniform free flow of material through the barrel thereby causing problems in the consistency of the plastic material. Other problems that can occur are galling in the barrel through adhesive wear or metal to metal contact and misalignment of the screw through excessive shear or deflection of the screw mechanism.
Previously, the only way to check the dimensional clearances in such a mechanism was to shut it down and remove the screw from the barrel. Since the plastic material would solidify, the screw and the barrel would have to be cleaned and the dimensions checked utilizing mechanical measuring devices. Then, the mechanism would have to be reassembled. Such a procedure tended to result in lost production time and did not completely eliminate the occasional breakdowns between regular measurements.
In the U.S. Pat. No. 4,604,251, there is shown an apparatus and method for checking the dimensional relationship between the screw flights and the inner diameter of a barrel or cylinder during the operation of the plastic material feed device. Utilizing the Foucault current or eddy method of detection, a probe is located in an aperture formed in the side of the barrel or cylinder. The end of the probe is positioned near the inner surface of the barrel or cylinder. The probe generates an electrical signal having a magnitude proportional to the distance between the outer edge of the flight on the screw and a sensing coil located in the probe. The probe output signal is sensed and converted to a digital distance display for use by the machine operator.
The magnitude of the signal can be scaled to generate a display number representing the actual measurement between the edge of the flight screw and the inside of the barrel or cylinder in English or metric units. The signal can be stored, selectively reset and two or more probe detection signals can be selectively displayed.